Retroflective sheeting

ABSTRACT

New methods for making &#34;embedded-lens&#34; retroreflective sheeting, and new forms of such sheeting, are provided. The new sheetings are retroreflective through an increased angular range, and also provide brighter overall retroreflectivity and other advantageous properties.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of earlier application Ser.No. 80,434 filed Oct. 1, 1979, and now abandoned.

BACKGROUND OF THE INVENTION

A typical prior-art "embedded-lens" retroreflective sheeting, asillustrated in FIG. 1 of the attached drawing, comprises a monolayer ofmicrospheres 11 embedded between transparent bonding and spacing layers12 and 13; a specularly reflective layer 14, typically aluminumvapor-deposited on the spacing layer 13; a layer of adhesive 15 coveringthe reflective layer; and a transparent top layer 16, which forms theexterior front surface of the sheeting. Light rays incident on thesheeting travel through the layers 16 and 12 to the microspheres 11,which act as lenses focusing the incident light approximately onto theappropriately spaced specularly reflective layer 14. Thereupon the lightrays are reflected back out of the sheeting along substantially the samepath as they traveled to the sheeting.

Embedded-lens sheeting as described has the advantage that, because themicrospheres are embedded within the sheeting, incident light rays arefocused onto the specularly reflective layer irrespective of whether thefront of the sheeting is wet or dry. Such sheeting was first taught inPalmquist et al, U.S. Pat. No. 2,407,680, and has been sold commerciallyfor many years in large volume and to the general satisfaction of itsusers. Despite that general utility, however, there has been a desirefor improvement in certain properties of the sheeting, one of which isin the angles at which the sheeting will retroreflect brightly.Retroreflection is generally at a maximum for light that isapproximately perpendicular to the sheeting (i.e., has an incidenceangle of 0°) and declines as the incident light slants away from theperpendicular (i.e., increases in incidence angle). Such a decline inretroreflectivity can be a disadvantage, for example, by limiting thelength of time that a retroreflective traffic-control sign is seen atnight at a vehicle passes by, or the length of time a retroreflectivelicense plate will be seen as two vehicles meet on the highway.

SUMMARY OF THE INVENTION

We have now found a new method for manufacturing embedded-lensretroreflective sheeting, as well as a new form of such sheeting, thatprovides a wider angular range of retroreflectivity, as well as otherimportant improvements in properties. Briefly, this new method comprisesthe steps of:

(1) continuously presenting a mass of microspheres against a movingcarrier web such as polyethylene-coated paper and heating the web so asto soften at least an exterior stratum of the web and partially embed adense monolayer of microspheres in the web;

(2) covering the monolayer of microspheres with a layer of transparentbinder material having an exterior surface cupped around themicrospheres;

(3) applying a specularly reflective layer to the cupped surface of thelayer of transparent binder material;

(4) in typical embodiments, applying an adhesive layer over thespecularly reflective layer;

(5) optionally removing the carrier web from the assembly formed by thelayer of transparent binder material and microspheres; and

(6) if the carrier web is removed, applying transparent polymer-basedmaterial over the microsphere-covered surface left upon removal of thecarrier web, e.g., by coating, laminating, dipping or spraying, to forma transparent top layer in which the microspheres are embedded.

Sheeting prepared by this method has a number of distinctive featureswhich are believed to account for significantly improved properties ofthe sheeting. These distinctive features can be illustrated by referenceto FIG. 2 of the attached drawing, which is an enlarged sectional viewthrough a representative sheet material of the invention 20.

(1) The front surfaces or edges of the microspheres 21 in sheeting ofthe invention tend to be more nearly aligned in a common plane than thefront surfaces of the microspheres 11 of the prior-art sheeting 10 shownin FIG. 1. Instead of front-surface alignment, the microspheres 11 inthe prior-art sheeting 10 tend to be aligned at their equator, sincethey were floated at their equator in the solution that forms thebonding layer 12 (surface-active agents are commonly applied asdescribed in Weber, U.S. Pat. No. 3,222,204, to assist the microspheresto float at their approximate equator).

The improved front-surface alignment of microspheres in sheeting of theinvention is theorized to occur during embedding of the microspheres inthe carrier web, as a result of the general planarity of the exteriorsurface of the carrier web, and the embedding of the microspheres intothat surface to a generally uniform depth. The microsphere surfacesembedded in the carrier web become the front surfaces in finishedsheeting of the invention.

The improved front-surface alignment of the microspheres contributes toan increased angular range of retroreflectivity, since a higherproportion of microspheres participates in the retroreflection at wideangles of incidence. As will be subsequently discussed, other featuresof sheeting of the invention are also believed to contribute to theimproved angularity, which is a significant increase. Whereas theretroreflection of prior-art embedded-lens sheetings generally declinesto about three-quarters of its maximum value when the incidence angleincreases to less than 35° (called the "three-quarter brightnessangle"), sheetings of the present invention retain three-quarterbrightness at angles of incidence beyond 35°, and in preferredembodiments exhibit three-quarter brightness angles that average about40° or more.

(2) Specularly reflective material 24 can overlie a larger proportion ofthe circumference of an individual microsphere 21 in sheeting of theinvention than it does in prior-art sheeting. This increased overlyingoccurs because the microspheres are generally embedded into the carrierweb to between 30 or 40 percent of their diameter, and in any event lessthan 50 percent of their diameter, to allow convenient removal from thecarrier web; and the spacing layer and specularly reflective layerapplied over the unembedded portions of the microspheres can penetratemore deeply between adjacent microspheres than they do in conventionallyprepared embedded-lens retroreflective sheeting. This greaterpenetration means that the specularly reflective layer extends into aposition to reflect light rays that impinge on the sheeting at higherincidence angles and thereby further contributes to the increasedangular range of sheeting of the invention.

(3) Microspheres are present in sheeting of the invention at a greaterdensity per unit of area than in the prior-art sheeting. The most commoncommercial varieties of retroreflective sheeting include sufficientmicrospheres (typically ranging between about 50 and 100 micrometers indiameter and averaging about 75 micrometers in diameter) to cover about60-65 percent of the area of the sheeting; the best prior-art product wehave ever seen covers about 72 percent of the area. In most sheeting ofthe invention microspheres cover at least 75 percent of the area of thesheeting and preferably cover at least 80 percent of the area. Theincreased concentration of microspheres appears to arise from the stopof presenting a mass of microspheres under pressure against athermosoftening carrier web such as polyethylene-coated paper andheating the web to embed the microspheres in the web.

The greater concentration of microspheres in sheeting of the inventionmeans that a higher percentage of incident light is focused onto thespecularly reflective layer and is thereby reflected back toward theoriginal source of the light. Whereas the most common commercialvarieties of sheeting as described have exhibited typicalretroreflective brightness on the order of 70 candella per square meterof sheeting per lux of incident light (or 70 candle power per squarefoot per lumen), sheetings of the invention are routinely capable of 100or more, and preferably achieve 150 or more candella per square meterper lux of incident light; these measurements are made under thestandard conditions of illuminating the sheeting at an incidence angleof -4° and measuring retroreflectivity at a divergence angle of 0.2°.

(4) Microspheres in sheeting of the invention are generally free of anychemical treatment for causing the microspheres to float in a layer ofbinder material. Such treatments generally operate by limiting theability of an organic liquid to wet or contact the microspheres, and itis believed that such a limitation may prevent full contact between themicrospheres and a binder material subsequently applied to them. Such alack of full contact would presumably interfere with optimum lighttransmission. It is theorized that the absence of such a chemicaltreatment provides improved optical contact between the binder materialand microspheres in sheeting of the present invention.

(5) Sheeting of the invention has good cupping of the spacing layerbehind the microspheres, thereby positioning a higher proportion of thespecularly reflective layer at the curved plane where light rays thatpass through an individual microsphere are focused. This improvedcupping is believed due to the greater penetration of the spacing layerbetween adjacent microspheres discussed above, and also to the use ofpolymeric materials or solutions of appropriate viscosity or flowcharacteristics. The improved cupping is indicated by measurements ofmicroroughness on the back of the spacing layer or on the back of thespecularly reflective layer coated on the spacing layer. Generally, inpreferred sheeting of the invention this cupping is sufficient formicroroughness readings of 125 microinches, arithmetic average, or more(about 3 micrometers or more) when measured on a Bendix portableProfilometer using a 2.5-micrometer-radius diamond stylus. The desiredmicroroughness varies somewhat with the size of the microspheres and canbe 5 or 10 percent higher for larger microspheres used in somereflective sheeting.

Prior-Art Statement

The first step in preparing reflective sheeting of the presentinvention, i.e., preparing a monolayer of microspheres partiallyembedded in a carrier web or support sheet, is the same as the firststep in preparing reflective sheeting taught in McKenzie, U.S. Pat. No.3,190,178. However, the rest of the method taught in the patent, and thereflective sheeting prepared by the method, known as encapsulatedsheeting, are fundamentally different from the method and embedded-lenssheeting of the invention; and the patent does not predict thesignificant improvement in wide-angle reflectivity achieved by sheetingof the invention.

The next steps in making encapsulated sheeting after the noted commonfirst step are to coat specularly reflecting material (rather thanbinder material as in the case of our new embedded-lens sheeting) ontothe microspheres while they are held in the carrier web, and then applybinder material over the specularly reflective layer. After drying ofthe binder material, the carrier web is stripped away, and a preformedsolid transparent cover film is laid against the exposed surfaces of themicrospheres. Heat and pressure are applied to the assembly along anetwork of interconnecting lines, softening and deforming the layer ofbinder material in the pressed areas into contact with the cover film.Upon cooling of the binder material, the cover film becomes adhered tothe assembly along the network of interconnecting lines, forming aplurality of hermetically sealed cells within which the microspheres areencapsulated and have an air interface.

The noted air interface of encapsulated sheeting achieves goodwide-angle reflectivity, because the microspheres partially protrudeabove the binder material in which they are embedded; but that conditiondoes not exist in embedded-lens sheeting of the invention, in which themicrospheres are fully embedded in transparent polymeric material.Encapsulated sheeting as described in the McKenzie patent has noadvantage in angularity of reflection over other reflective sheetings inwhich microspheres have an air interface, and the McKenzie patent offersno reason to think that a method starting in the same way as the methodfor preparing encapsulated sheeting would provide increased angularityin embedded-lens sheeting.

Another difference between our invention and prior-art teachings aboutencapsulated sheeting is a difference in rationale or purpose for usinga carrier web in which microspheres are temporarily embedded. Use of acarrier web that may be stripped away to leave partially protrudingmicrospheres is a natural step in manufacture of encapsulated sheeting,since a microsphere-exposed surface is desired in the final product. Bycontrast, in sheeting of the present invention, the microspheres areintended to be finally embedded in a binder material. Rather than beingsimply a support against which a final microsphere-exposed surface ofsheeting is prepared, as in the case of encapsulated sheeting, we usethe carrier web as a tool to desirable structural features such asfront-surface alignment, increased coverage of specularly reflectivematerial over the back surface of the microspheres, and dense packing ofmicrospheres.

Air-exposure of microspheres has also characterized other instances inwhich microspheres are temporarily embedded in a carrier web--seePalmquist et al, U.S. Pat. No. 2,963,378 (microspheres arehemispherically reflectorized while held in the carrier web and thenremoved to form a free-flowing mass of reflective elements); Palmquistet al, U.S. Pat. No. 3,382,908 (an exposed-lens elastomeric sheetinguseful for application to sidewalls is prepared by reflectorizing themicrospheres held in the carrier web, coating elastomeric bindermaterial over the microspheres, and removing the carrier web); andHarper et al, U.S. Pat. No. 4,102,562 (transfer sheet material forforming reflectorizing images on a substrate is formed by reflectorizingmicrospheres while they are held in a carrier web and then printing overthe microspheres in an imagewise pattern with a transfer layer; with thecarrier web still present, the transfer layer is activated and adheredto a substrate, whereupon the carrier web is stripped away to leaveexposed-lens reflectorized images on the substrate).

Other structural differences between sheeting of the present inventionand the prior-art encapsulated sheeting include the use of a transparentspacing layer between the microspheres and specularly reflective layerin sheeting of the present invention, while in the prior-artencapsulated sheeting the binder material is generally pigmented and isthermoformable to form the network of bonds to a cover sheet; andmicrospheres of different indices of refraction are used in the twosheetings (approximately 1.9-index microspheres in the prior-artencapsulated sheeting and approximately 2.2-index microspheres insheeting of the invention).

BRIEF DESCRIPTION OF THE DRAWINGS

As previously indicated,

FIGS. 1 and 2 are enlarged sectional views through, respectively, aprior-art reflective sheeting and a reflective sheeting of theinvention;

FIG. 3 is a schematic diagram of a portion of the apparatus used inmanufacturing sheeting of the invention;

FIGS. 4-6 are enlarged sectional views through sheet structures preparedin the course of making a sheeting of the invention; and

FIG. 7 is an enlarged sectional view through a different variety ofsheeting of the invention.

DETAILED DESCRIPTION

Additional description of the invention will first be provided byrecitation of an exemplary preparation of sheeting of the invention,using FIGS. 2-6 for reference.

EXAMPLE 1

A carrier web 28 comprising a base sheet of paper 29 coated on one sidewith a 25-to-50-micrometer-thick layer 31 of low-density polyethylene ispassed over heating rolls 37 and 38 heated respectively to 195° and 220°F. A bank 39 of microspheres 21 having a refractive index of 2.26 and adiameter ranging from 65 to 85 micrometers is maintained next to thelast heating roll, and a dense monolayer of microspheres becomes adheredto the polyethylene layer on the carrier sheet. The softenedpolyethylene surface is rather slippery, which may contribute to a densepacking of the microspheres adhered to the web. Thereupon the web ispassed through ovens heated to about 280° F., whereupon the polyethylenefurther softens, capillates up the microspheres, and draws themicrospheres into the polyethylene until the microspheres are embeddedto between 30 and 40 percent of their diameter. During the originaladherence of the microspheres to the carrier web, the microspheres arebelieved to become embedded to approximately the same depth, and thesubsequent capillation of the polyethylene appears to leave themicrospheres embedded at an approximately common depth. From visualinspection of several small spots of the web, it was found that themicrospheres covered about 75 percent of the area of the web.

Next 100 parts of a 25-percent-solids solution of aliphatic urethaneresin in a one-to-one mixture of isopropanol and toluol solvents(Permuthane U-6729 supplied by Beatrice Chemical) was mixed with 7 partsof ethylene glycol monomethylether and the mixture coated over themicrosphere-covered surface of the carrier web with a bar-coater. Themixture had a coating viscosity of about 6000 centipoises. The coatingwas then dried for 4-5 minutes in forced-air ovens heated to 125°F.-250° F. to leave a transparent spacing layer 23. The coated materialwas found to have cupped around the back surfaces of the microspheres inthe manner illustrated in FIG. 5 sufficiently for the back surface ofthe sheeting to have a microroughness of at least 160 microinchespeak-to-peak as measured by a portable Bendix Profilometer having a2.5-micrometer-diameter stylus with a maximum stylus pressure of 1.5grams.

A layer 24 of aluminum was then vapor-coated onto the cupped surface ofthe spacing layer 23 by a known procedure to leave a sheet material asshown in FIG. 5.

A layer 25 of pressure-sensitive acrylate adhesive was then coated fromsolution over the vapor-coated aluminum and dried, after which thecarrier web 28 was stripped away, leaving a sheet material 33 as shownin FIG. 6 in which the microspheres 21 partially protruded from thefront surface of the sheet material.

Thereafter an oriented polymethylmethacrylate film 26 coated with alayer of pressure-sensitive acrylate adhesive 27 was pressed against themicrosphere-exposed surface, embedding the microspheres into theadhesive and forming a transparent front layer on the sheeting in themanner shown in FIG. 2. The coated film may be heated somewhat duringlamination (to temperatures less than the temperature of orientation ofthe film) to improve adhesion.

EXAMPLE 2

Example 1 was repeated except that a preformed film comprising a linearsaturated polyester resin understood to be the reaction product ofterephthalic acid, 1,2-cyclohexanedicarboxylic acid, ethylene glycol,diethylene glycol, and cyclohexanedimethanol (Bostick 7979 resin made byUSM Corp. of Middleton, Mass.) was laminated to a web as shown in FIG. 4to form the spacing layer 23. This lamination operation was performed bypassing the assembly through heated nip rolls, one of which was a softelastomeric roll. This operation provided a spacing layer ofsubstantially uniform thickness over its whole extent, and was followedby the remaining steps described in Example 1.

EXAMPLE 3

Example 1 was repeated except that instead of adhering a top film 26into the sheeting, a transparent layer of pressure-sensitive acrylateadhesive was coated over the partially exposed microspheres. In use,this sheeting can be adhered to a transparent panel such as anautomobile window to form a retroreflective label viewable through thepanel. If desired, a message is printed onto the microsphere surfaceprior to application of the adhesive. Removal of the label may disruptthe optical system, i.e., by loosening or removing microspheres,whereupon transfers of the sheeting are revealed due to nonuniformretroreflection.

EXAMPLE 4

Example 1 was repeated except that after the carrier web 28 was strippedaway, the sheeting was adhered to a metal plate and the plate embossedto form a license plate. The plate was then dipped into a standardlicense plate coating solution, and the coating dried and cured, therebycompleting the optical system and making the plate reflective whetherwet or dry.

A wide variety of kinds of materials may be used in preparing sheetmaterials of the invention. The spacing layer 23 or top layer 26 may bean acrylic resin, alkyd resin, polyurethane resin, polyester resin,polyvinyl butyrate, or combinations of such resins. These resins may beapplied from solution or dispersion or from liquids that contain novolatiles. The materials may be nonreactive or may react to across-linked relatively insoluble and infusible state.

The thickness of the spacing layer 23 will depend on the ratio of theindex of refraction of the microspheres to the index of refraction ofthe top layer. The layer is sufficiently thick so as to position thespecularly reflective layer 24 at the approximate focal plane for lightrays passing through the microspheres. If the ratio reachesapproximately 1.9 through an appropriate combination of high-indexmicrospheres and low-index top layers, no spacing layer is needed, and aspecularly reflective layer may be applied directly to the microspheres.

Instead of forming the specularly reflective layer 24 from metal,dielectric coatings taught in Bingham, U.S. Pat. No. 3,700,305, can beused. Also, specularly reflective pigment may be added to a layer suchas the layer 25 in FIG. 2 instead of using a specularly reflective layersuch as the layer 24.

Another structural form for sheeting of the invention is shown in FIG.7. This sheeting includes an additional layer 35 of binder material,which is useful to perform such additional functions as providing addedstrength to the construction, or introducing color variations. It shouldalso be noted that although in most cases the microspheres are initiallydeposited or embedded into a removable carrier web, the carrier web canbe left as a permanent part of the sheeting if it has desirable opticalproperties. For example, a film of polyethyleneterephthalate coated witha thermosoftening polymer may be used.

In most sheeting of the invention the microspheres average less than 100micrometers in diameter. The narrower the range of diameters, the moreuniform and better the properties of the sheeting. Preferably, the rangeof diameters of the microspheres will extend beyond the average diameterby no more than about plus-or-minus 20 percent, and more preferably byno more than about plus-or-minus 10 percent.

Retroreflective sheeting of the invention reflects most brightly whenthe top layer is uncolored and clear, in which case the sheeting willgenerally have a silver or gray appearance caused by the metallicappearance of the vapor-coated aluminum. Colored sheetings can beprepared by placing dyes or transparent pigments in the spacing layer,in the additional layer 35 as shown in FIG. 7, or in the top layer. Theinvention offers manufacturing economies, since a base material such asshown in FIG. 6 may be prepared, and different top layers laterlaminated or otherwise applied to the base material.

Sheeting of the invention is generally sold in roll form with thesheeting wound upon itself, and lengths of sheeting are unwound and cutfrom the roll as needed to cover a sign substrate or other surface. Anyrelease liner covering the adhesive surface of the sheeting is removedwhen the sheeting is adhered to a substrate.

What we claim is:
 1. Component sheet material useful in combination witha transparent layer applied in full contact with the front surface ofthe sheet material to complete a wet-or-dry retroreflective sheetingthat is retroreflective at its front surface only, said sheet materialbeing wound upon itself in a storage roll and comprising a binder layerand a monolayer of transparent microspheres partially embedded in andpartially exposed above the binder layer, whereby the exposed portionsof the microspheres become embedded in the transparent layer uponapplication of the layer to the sheet material; the microspheres beingpresent in an amount sufficient to cover at least 75 percent of the areaof the binder layer, being exposed above the binder layer by on theaverage less than 50 percent of their diameter, and being arranged withtheir front surfaces in substantial alignment, whereby the completedretroreflective sheeting exhibits a three-quarter-brightness angle of atleast 35°; and silver versions of the completed retroreflective sheetingexhibit a retroreflective brightness at an incidence angle of -4° anddivergence angle of 0.2° of at least 100 candella per square meter ofsheeting per lux of incident light.
 2. Sheet material of claim 1 inwhich the microspheres are free of chemical coatings that limit theability of organic polymeric binder materials to wet the microspheres.3. Sheet material of claim 1 in which the binder layer is cupped aroundthe back surface of the microspheres sufficiently for the back surfaceof the layer to have a microroughness of at least 120 micrometerspeak-to-peak.
 4. Sheet material of claim 1 in which the binder layerincludes a specularly reflective layer separated from the microspheresby a transparent spacing layer in which the microspheres are embedded.5. Sheeting retroreflective under either wet or dry conditions woundupon itself in a storage roll and comprising a binder layer and amonolayer of transparent microspheres partially embedded in andpartially protruding above the binder layer, a transparent layerdisposed against the front surface of the binder layer, with theprotruding portions of the microspheres embedded in the transparentlayer; the microspheres being present in an amount sufficient to coverat least 75 percent of the area of the binder layer, protruding abovethe binder layer by on the average less than 50 percent of theirdiameter, and being arranged with their front surfaces in substantialalignment, whereby the sheeting exhibits a three-quarter-brightnessangle of at least 35°; and silver versions of the sheeting exhibit aretroreflective brightness at an incidence angle of -4° and a divergenceangle of 0.2° of at least 100 candella per square meter of sheeting perlux of incident light.
 6. Sheeting of claim 5 in which the microspheresare free of chemical coatings that limit the ability of organicpolymeric binder materials to wet the microspheres.
 7. Sheeting of claim5 in which the binder layer is cupped around the back surface of themicrospheres sufficiently for the back surface of the layer to have amicroroughness of at least 120 microinches peak-to-peak.
 8. Sheeting ofclaim 5 in which the binder layer includes a specularly reflective layerseparated from the microspheres by a transparent spacing layer in whichthe microspheres are embedded.
 9. Sheeting of claim 5 in which themicrospheres cover at least 80 percent of the area of the binder layer.10. Sheeting of claim 5 which exhibits a three-quarter-brightness angleof at least 40°.
 11. Sheeting retroreflective under either wet or dryconditions wound upon itself in a storage roll and comprising a binderlayer and a monolayer of transparent microspheres partially embedded inand partially protruding above the binder layer, a transparent layerdisposed against the front surface of the binder layer, with theprotruding portions of the microspheres embedded in the transparentlayer; the microspheres being present in an amount sufficient to coverat least 75 percent of the area of the binder layer and protruding abovethe binder layer by on the average less than 50 percent of theirdiameter; the binder layer being cupped around the back surfaces of themicrospheres; the sheeting exhibiting a three-quarter-brightness angleof at least 35°; and silver versions of the sheeting exhibiting aretroreflective brightness at an incidence angle of -4° and a divergenceangle of 0.2° of at least 100 candella per square meter of sheeting perlux of incident light.
 12. Sheeting of claim 11 in which themicrospheres are free of chemical coatings that limit the ability oforganic polymeric binder materials to wet the microspheres.
 13. Sheetingof claim 11 in which the binder layer is cupped around the back surfaceof the microspheres sufficiently for the back surface of the layer tohave a microroughness of at least 120 microspheres peak-to-peak. 14.Sheeting of claim 11 in whch a specularly reflective layer is coated onthe cupped surface of the binder layer.
 15. Retroreflective assemblycomprising a substrate having an extensive surface, and a continuousretroreflective sheeting adhered over said surface by an adhesive layercarried on the sheeting, said sheeting comprising a binder layer, amonolayer of transparent microspheres partially embedded in andpartially protruding above the binder layer, and a transparent layerdisposed against the front surface of the binder layer, with theprotruding portions of the microspheres embedded in the transparentlayer; the microspheres being present in an amount sufficient to coverat least 75 percent of the area of the binder layer and protruding abovethe binder layer by on the average less than 50 percent of theirdiameter, and the binder layer being cupped around the back surfaces ofthe microspheres; the sheeting exhibiting a three-quarter-brightnessangle of at least 35°; and silver versions of the sheeting exhibiting aretroreflective brightness of an incidence angle of -4° and a divergenceangle of 0.2° of at least 100 candella per square meter of sheeting perlux of incident light.
 16. Retroreflective assembly of claim 15 in whichthe binder layer in the retroreflective sheeting includes a specularlyreflective layer separated from the microspheres by a transparentspacing layer in which the microspheres are embedded.
 17. Retrorefleciveassembly of claim 15 in which the microspheres cover at least 80 percentof the area of the binder layer.
 18. Retroreflective assembly of claim15 in which said sheeting exhibits a three-quarter-brightness angle ofat least 40°.
 19. Vehicular license plate comprising a substrate andretroreflective sheeting adhered to the substrate, said sheetingcomprising a binder layer and a monolayer of transparent microspherespartially embedded in and partially protruding above the binder layer, atransparent layer disposed against the front surface of the binderlayer, with the protruding portions of the microspheres embedded in thetransparent layer; the microspheres being present in an amountsufficient to cover at least 75 percent of the area of the binder layerand protruding above the binder layer by on the average less than 50percent of their diameter; the binder layer being cupped around the backsurfaces of the microspheres; the sheeting exhibiting athree-quarter-brightness angle of at least 35°; and silver versions ofthe sheeting exhibiting a retroreflective brightness at an incidenceangle of -4° and a divergence angle of 0.2° of at least 100 candella persquare meter of sheeting per lux of incident light.